Modern cosmology tells us that the universe started 13.8 billion years ago as a point-like source [singularity] that expanded into the cosmos we know today. It’s perhaps not surprising that the Big Bang, the mother of all explosions, made a sound. We know this from measurements of the cosmic microwave background (CMB) made by the Wilkinson Microwave Anisotropy Probe (WMAP) and Planck space missions. Just after the Big Bang, all the matter in the universe was a superhot plasma confined to the relatively small space of the emerging universe. In that closed space there were vibrations, sound waves of compression and rarefaction that propagated throughout the plasma. The sound persisted as the universe expanded.
About 300,000 years after the Big Bang, the cooling and expanding universe reached the point where electrically neutral hydrogen atoms could form from the free electrons and protons of the hot plasma. When this happened, light, which had been trapped
